1. Field of the Invention
The present invention relates to an image sensing apparatus capable of confirming exposure state during still image photography and a control method for the image sensing apparatus, and an image sensing system.
2. Description of the Related Art
With a conventional film photograph, exposure state is not known until the film is developed. However, with a digital camera the exposure state can be confirmed then and there.
Japanese Patent Laid-Open No. 2005-117396 discloses a technology capable of confirming the exposure state during photography even at long exposure times. In this technology, certain pixels forming a portion of an image sensor are read out during the exposure period at predetermined time intervals using destructive read out and the read out pixel data is respectively added on the previously read out pixel data and the added pixel data is displayed.
Japanese Patent Laid-Open No. 2002-27326 discloses a technology capable of displaying the exposure state during main exposure by using a non-destructive read out type image sensor.
In order to observe the exposure state in still image photography at slow shutter speed, the image signals could be read out during exposure and a digitized image added digitally. However, by dividing the read-out over multiple times, fixed-pattern noise and random noise generated in the read-out circuit are amplified, degrading the quality of the still image. Moreover, there is a possibility that exposure light amount per time may be inadequate and an image not obtained because image signals are obscured by quantification error.
In addition, in the method described in Japanese Patent Laid-Open No. 2005-117396, an image for exposure state display is susceptible to the problem encountered with digital addition described above, and moreover, in exposure state display, because pixels at the same location are always used there exist pixels that are not displayed. Conversely, with respect to still images, there is the problem of drop-out of information of the image used for exposure state display.
As a method that adds analogically instead of adding digitally, non-destructive read out from the image sensor is possible. Non-destructive read out is easy with a CMOS configuration, but in order to make the CMOS configuration low-noise high performance, it is necessary to carry out the destructive read out that completely transfers the charge from photodiodes.